Due to increase in communication traffic of recent years, demand for an optical communication transmitting device has been grown. The optical communication transmitting device is actively introduced not only in an optical relay node introduced in a backbone network, but also in a local network recently, and further, an optical network is formed in a subscriber system. In this manner, the optical communication system has an important role in a global information network. An optical amplifying relay provided with a wavelength multiplexing optical amplifier such as an erbium doped fiber amplifier (EDFA) for each transmission path for realizing high-capacity and long-distance transmission at a low cost and high confidence predominates in the optical network.
In the optical amplifying relay system, with a condition that a relay loss is large due to a long transmission path, an input level of signal light to the optical amplifier becomes smaller, so that it is possible that SN (ratio of signal light power to noise light power) deteriorates and transparency characteristics deteriorate. In order to avoid this, a transmission path distributed Raman amplification in which excitation light (pump light) is injected into the transmission path to use an amplification effect using an effect of induction Raman scattering is effectively applied.
A distributed Raman optical amplifier (DRA) has been already put to practical use as an effective realizing means for providing the transmission path distributed Raman amplification. By applying the distributed Raman amplifier, the input level to the to optical amplifier such as the EDFA increases to increase the SN, and the transmission characteristics are improved, so that the number of spans to be transmitted increases. In addition, in the Raman amplification, the wavelength characteristics of gain may be stabilized by performing automatic gain control.
Japanese Patent Application Laid-Open No. 2004-193640 to be described later discloses the conventional technology regarding the automatic gain control of the DRA. Japanese Patent Application Laid-Open No. 2004-193640 discloses the technology to keep the output wavelength characteristics of the Raman amplification constant by providing the transmission path distributed Raman amplification with the automatic gain control regardless of conditions such as the gain efficient of an optical fiber forming the transmission path and a loss of connection path, and the signal light level. The technology disclosed in Japanese Patent Application Laid-Open No. 2004-193640 refers to obtain the Raman gain by using the reference light not subjected to the Raman amplification and control the pump light such that this reaches a predetermined value.
However, if the condition of the gain reference light is such that this is not subjected to the Raman amplification, the wavelength of the reference light is set in a range very distant from the wavelength band of the signal light. Then, the characteristics such as the loss coefficient of the optical transmission path differ between the signal light and the reference light, so that a significant error occurs between the level of the signal light and the level of the reference light at an output point of the optical transmission path.
Therefore, even if the pump light is controlled based on the level of the reference light detected at the output point, it is difficult to realize the automatic gain control of the DRA with high accuracy, and it is difficult to compensate the gain wavelength characteristics of the DRA by a fixed gain equalizer regardless of change of a kind of fiber and fiber characteristics of the optical transmission path.